1. Field of the Invention
The present invention relates to a transmission bus system including a plurality of stations linked to the transmission bus in the form of multidrop, and particularly to a remote repeater installed for the purpose of extending the distance of a segment of the transmission bus.
2. Description of the Prior Art
A conventional remote repeater used in a multidrop transmission bus system is disclosed in a publication entitled, "The Ethernet: A Local Area Network, Data Link Layer and Physical Layer Specifications", Version 1.0, Sept. 30, 1980. In FIG. 6 is shown a transmission system in which the conventional remote repeaters are used, including a plurality of segments (four segments in this case) 13, 23, 33 and 43 each consisting of a twisted pair cable or coaxial cable. Each segment has a connection of two stations 14 and 15 (24 and 25; 34 and 35; 44 and 45) capable of transmission and reception over the transmission bus. Provided at both ends of segments 13, 23, 33 and 43 are terminating resistors 11 and 12; 21 and 22; 31 and 32; 41 and 42, respectively, for matching the transmission line impedance. Two adjacent segments 13 and 23 are linked by a signal line 51 on which remote repeaters 17 and 26 are installed. Similarly, the segments 23 and 33 are linked via a signal line 52 with remote repeaters 27 and 36 on the signal line 52, and the segments 33 and 43 are linked via a signal line 53 with remote repeaters 37 and 46 on the signal line 53. The remote repeaters 17, 26, 27, 36, 37 and 46 are substantially the same in their structure, and the following description for the remote repeater 17 is also applied to the remaining ones. The arrangement of the remote repeater 17 shown in FIG. 7 includes receivers 61 and 62 for receiving signals on the segment 13 and signal line 51, respectively, drivers 63 and 64 for outputting signals onto the segment 13 and signal line 51, respectively, carrier detecting circuits 65 and 66 for detecting carriers on the lines 13 and 51, respectively, a decision circuit 69 for testing the order of carrier detections, AND gates 70 and 71, and waveform shaping circuits 67 and 68 for shaping the signals at the output of the receivers 61 and 62, respectively. The receiver 61 (62) and driver 63 (64) in combination constitute a bidirectional driver/receiver.
Next, the operation of the remote repeater will be described on the assumption that the station 14 transmits a signal. In the repeater 17, the receiver 61 receives the signal on the segment 13 and the carrier detecting circuit 65 detects the carrier in the signal. The decision circuit 69 responds to the reception of the carrier from the segment 13 to enable the AND gate 70. Then, the signal received by the receiver 61 is shaped by the waveform shaping circuit 67 and is fed through the AND gate 70, which sends out it through the driver 64 over the signal line 51. The other repeater 26 receives the signal on the signal line 51 and outputs the shaped signal to the segment 23, and other stations 24 and 25 can receive the signal transmitted by the station 14. The remaining repeaters 27, 36, 37 and 46 operate in the same manner and all of the remaining stations can receive the signal from the station 14.
A problem is that the signal provided by the driver 64 of the remote repeater 17 is directly received by the receiver 62, causing the carrier detecting circuit 66 to respond to it. However, the decision circuit 69 operates to detect which of the carrier detecting circuits 65 and 66 has provided an active output in producing its output, and therefore the signal line 13 is not disturbed by the operation of the driver 63. In this case, as shown in the timing chart of FIG. 8, the AND gate 70 is enabled, while the AND gate 71 is disabled. When the station 14 has ceased transmission, the carrier detecting circuit 65 ceases the active output, causing the AND gate 70 to be disabled, preparatory to the subsequent carrier detection.
Next, in case that the station 24 transmits the signal, the stations 14, 15, 34, 35, 44 and 45 can receive the signal transmitted from the station 24 through the remote repeaters 17, 26, 27, 36, 37 and 46. The following describes specifically the operation of the remote repeater 17. As opposed to the previous operation, the receiver 62 receives the signal on the signal line 51, the carrier in the signal is detected by the carrier detecting circuit 66, which provides the output to the decision circuit 69. The circuit 69 enables the AND gate 71 so that the signal shaped by the waveform shaping circuit 68 is outputted through the driver 63 over the signal line 13, and the stations 14 and 15 can receive it.
The conventional remote repeater arranged as described above needs the use of remote repeaters in pairs, and therefore the construction cost becomes extremely high when the system includes many segments. For example, a transmission system having four segments, as in the construction of FIG. 6, necessitates six remote repeaters, and if the number of segments increases to eight, the number of remote repeaters increases to 14.